Metastatic phenotype: Growth factor dependence and integrin expression

Homogeneous subpopulations, which are endowed with low or high metastatic potential, were selected from Lewis lung carcinoma (3LL) in an attempt to correlate metastatic phenotype with specific properties of tumor cells. Since the growth of malignant cells at secondary sites could depend on their ability to respond to microenvironments, the growth factor dependence of 3LL variants has been studied. The ability of variant lines to grow in monolayer and in soft agar cultures, either in the presence or absence of different growth factors or serum, was analyzed and correlated with their metastatic potential. The reported results demonstrate that tumor cells expressing higher metastatic potential also exhibit higher capacity to grow and proliferate in all the culture conditions tested, independently of the addition of exogenous growth factors or serum. Moreover, since highly metastasizing cells express a significant amount of TGF-β(1) mRNA, a pattern of autocrine growth is postulated for 3LL metastatic cells. One relevant aspect of the phenotype of transformed cells is their reduced adhesion to solid substrates; this phenomenon is thought to reflect the invasive and metastatic potential of tumor cells. Since the adhesion of the cells to substrata is mediated by molecules of the extracellular matrix, the expresssion of extracellular matrix receptors (integrins) was studied on 3LL metastatic variants. In particular, through immunochemical and biochemical studies we investigated the expression of the laminin receptor (α6/β1) and of a novel receptor (integrin: α6/β4), of unknown function. The receptors were quantitated on the cell surface of 3LL variants by the use of specific monoclonal antibodies which recognize, respectively, different epitopes of α6,β4 or β1 subunits. Results demonstrate that the novel integrin α6/β4, is specifically expressed in highly metastasizing 3LL cells, whereas the laminin receptor α6/β1 is expressed in all 3LL variants. In conclusion, data presented demonstrate that 3LL cells endowed with higher metastatic potential are more independent of the microenvironmental conditions in that they possess a higher autocrine capacity than the lower metastasizing ones, and could acquire higher capacity to invade through the expression on their cell surface of specific receptors for cell adhesion (the novel integrin, defined as α6/β4).